Intramolecular cyclizations of Co2(CO)6-complexed propargyl radicals:: Synthesis of d,l- and meso-1,5-cyclodecadiynes

Intramolecular radical cyclizations of Co-2(CO)(6)-complexed bis-propargyl alcohols 1-5 provide an easy, two-step access to the otherwise hardly accessible d,l- and meso-3,4-diaryl-1,5-cyclodecadiynes 21-25. The acid-induced generation of bis-cations 6-10 is followed by reduction with a 100-fold excess of zinc, generating key reactive intermediates, i.e. the cobalt-complexed bis-propargyl radicals 11-15. The substitution pattern (0-, 4-, 3,4-, 3,4,5-) and the nature of aromatic substituents (H, i-Pr, OMe) are found essential both to the diastereoselectivity of the process-with d,l:meso ratios varying from 54:46 to 80:20-and the isolability of individual stereoisomers. In particular, an accumulation of methoxy groups on the periphery of the aromatic rings facilitates separation in both intra- and intermolecular reactions. The observed disparity in the level of diastereoselection in intra- and intermolecular reactions is accounted for on the basis of conformational analysis, calculation data for cobalt-complexed propargyl radicals, and the concept of a CH/pi coordination. The new knowledge thus acquired has a predicting power, allowing future substrates to be designed, both topologically and stereoelectronically, in a manner favoring either d,l or meso diastereomers.